Isaac makes its final approach towards Louisiana

By: Dr. Jeff Masters , 3:29 PM GMT on August 28, 2012

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The winds and water are rising all along the coast from Louisiana to the Florida Panhandle as Tropical Storm Isaac makes its final approach. Two hurricane hunter aircraft in the storm are measuring a steadily lowering pressure and increasing winds aloft, but hurricane-force winds have not yet been observed at the surface. The 8:30 am center fix found a pressure of 976 mb, which is very low for a tropical storm. Top surface winds measured with the SFMR instrument were 70 mph, but the plane measured 102 mph at an altitude of 5,000 feet. It's more typical to see surface winds of 85 mph with a storm with these characteristics. Infrared and visible satellite loops and hurricane hunter reports from this morning have shown that Isaac has developed a 25-mile diameter eye, though the eyewall has not yet formed a full circle around the eye. Heavy thunderstorm activity is lacking on the north side, where light wind shear of 5 -10 knots is still pumping some dry air into the circulation.


Figure 1. Morning satellite image of Isaac. Note how dry air has wrapped into the west side of the storm, causing a lack of heavy thunderstorm activity.

Isaac's rains
One of the most remarkable features of Isaac has been the huge spiral band that parked itself along most of the east coast of Florida and remained there for an entire day, despite the fact the center of the storm moved 400 miles away. This rain band was amplified by a weak trough of low pressure along the East Coast, which pulled away from the coast Monday night, taking the band of heavy rain out to sea (except for a few lingering showers near West Palm Beach.) Isaac's heaviest rains fell along a swath from the east coast of Florida near West Palm Beach to the center of the state, just south of Orlando. The 2-day rainfall total of 9.03" at West Palm Beach brought their monthly rainfall total to 22.28", a new August record (old record: 20.12" in 1995.) Vero Beach's 6.48" of rain was a record for any August day. A possible tornado touched down there, damaging 20 mobile homes. In the Keys, rainfall totals as high as 7.94" (at Upper Matecumbe Key) were measured. Heavy rains from Isaac are lingering over Cuba but have ended in Haiti and the Dominican Republic; flash floods in Haiti from Isaac's torrential rains killed at least 24, and two died in the Dominican Republic. The big concern in Haiti is the heavy damage that was done to crops, and the likelihood that the storm's rains will worsen the cholera epidemic that has killed over 7,000 Haitians.


Figure 2. Radar-estimated rainfall from Miami, Florida radar shows that Isaac has dumped a wide swath of 8+ inches of rain (orange colors) across the state. Rainfall amounts in excess of 20" may have fallen just west of West Palm Beach, though the highest amount reported by a rain gauge was 13.10" at Greenacres in Palm Beach County.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are fairly unified taking Isaac ashore near Southeast Louisiana late Tuesday night or early Wednesday morning, but continue to show some differences in what happens after that. Isaac may scoot nearly west-northwest just inland along the coast into Texas, as predicted by the ECMWF model, or head straight inland to the northwest and into Arkansas, as predicted by the GFS model. The latest 8-day precipitation forecast from the GFS model calls for 10 - 20 inches of rain over much of Louisiana. It appears likely that Arkansas will see some heavy rains of up to five inches, which would help put a dent in the exceptional drought conditions there.

Intensity forecast for Isaac
Low wind shear of 10 knots or less is likely until landfall, along with very warm ocean temperatures. An analysis of upper level winds from the University of Wisconsin CIMSS shows that Isaac's upper-level outflow is the strongest we've seen, with a solid outflow channel to the south. These conditions favor continued strengthening of Isaac until landfall. However, we've observed in the past many instances of hurricanes suddenly weakening in the final 12 hours before making landfall along the Central Gulf Coast. Katrina, Gustav, Dennis, Ivan, and Rita all did so. A July 2012 paper in the Bulletin of the American Meteorological Society by Rosenfeld et al. titled, AEROSOL EFFECTS ON MICROSTRUCTURE AND INTENSITY OF TROPICAL CYCLONES, theorizes that this may happen because of the impact of small particles that get pulled into the outer circulation of hurricanes, seeding the clouds. These small particles, primarily from air pollution, serve as the seed around which water condenses, increasing the rain in the outer spiral bands. The increase in rain and heat energy at the periphery of the storm comes at the expense of the eyewall and inner core, where the winds tend to weaken. A detailed modeling study by Khain et al. (2010) of Hurricane Katrina in the final day before landfall was able to reproduce the storm's weakening only when this air pollution effect was included. This impact of small particles on hurricanes is not included in any operational hurricane model.


Figure 3. Tide gauge data from Shell Beach, located in Lake Borgne just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Storm surge observations from Isaac
Isaac's storm surge has peaked along the west coast of Florida. As I explain in our Storm Surge Tutorial, we are most interested in the storm tide--the height above Mean Sea Level (MSL) of the tide plus the storm surge. The storm tide is the number given in NHC advisories for how much above ground level the ocean will be at the coast. The storm surge is the extra elevation of the water due to wind blowing on the water, and does not include the action of waves on top of the water, nor the tide. Tide gauges are specially constructed so that transient waves do not impact water level measurements. At Cedar Key on the West Florida coast north of Tampa, a storm surge of 3' and storm tide of 3.8' were observed early this morning. These were the highest water levels measured at any tide gauge along the Florida west coast. Higher storm surges are occurring in the Florida Panhandle. As of 9 am EDT, here were the storm surge/storm tide measurements along the Florida Panhandle:

Apalachicola, FL: 3.5' storm surge, 4' storm tide
Panama City, FL: 2.3' storm surge, 3.3' storm tide
Pensacola, FL: 1.5' storm surge, 2.5' storm tide

A storm surge of 3.5 feet was recorded at 10 am EDT at Shell Beach on the east side of New Orleans in Lake Borgne. This site will have one of the highest surge values during Isaac; a storm surge of 9.5' was measured at Shell Beach during Hurricane Gustav in 2008.


Figure 4. Category 2 Hurricane Gustav of 2008 followed a very similar path to Isaac, and brought a storm tide (the combined effect of the storm surge and tidal levels) of up to 14.5' above ground level to the east side of New Orleans. Isaac's surge may be similar, though probably a little less, than Gustav's.


Figure 5. Track of Hurricane Gustav of 2008, which followed a path very similar to that of Isaac's predicted path.

Isaac: similar to Category 2 Hurricane Gustav of 2008 in destructive power?
Isaac is a huge and slow-moving storm, with tropical storm-force winds that extend out 205 miles from the center. Isaac has cut its forward speed down from 14 mph yesterday to 10 mph today, and a large swath of the coast will be subject to high winds and a large storm surge for an usually long period of time for a hurricane--up to 24 hours. Long duration winds are much more damaging than short duration winds, and a long duration storm surge event allows damage to occur during multiple high tide cycles. The long duration storm event will also allow very high rainfall totals, resulting in greater fresh-water flooding problems than usual. As a result, I expect Isaac's to cause more damage than the typical Category 1 hurricane. The 9:30 am EDT Integrated Kinetic Energy analysis from NOAA's Hurricane Research Division put the destructive potential of Isaac's winds near 2.3 on a scale of 0 to 6, but the destructive potential of Isaacs's storm surge was 4.1 on a scale of 0 to 6. For comparison, the storm surge destructive potential of Category 2 Hurricane Gustav of 2008 was rated at 4.2 on a scale of 0 to 6, and the wind destructive potential was 1.1--which is lower than Isaac's, even though Isaac was just a tropical storm at 9:30 am EDT. Gustav brought a storm tide (the combined height of the storm surge and high tide) of 14.5' to the east side of New Orleans, and 11' to Waveland, Mississippi. However, the destructive potential of Isaac's surge may be overrated by this analysis. Wave heights this morning from buoy and ships in Isaac have mostly been below 15', which is quite unimpressive. One ship report to the SE of the storm had a 19' wave height (thanks to meteorologist Steve Gregory for pointing this out.) With only another 12 - 18 hours over water, Isaac likely won't have time for its slowing increasing winds to build up a storm surge that will reach as high as 14', like Gustav did. The official NHC forecast of maximum storm surge height of 12' looks like a good one. The highest rainfall total observed in Gustav was 21" at Larto Lake, Louisiana, and I expect we'll exceed that for Isaac, since the storm is moving more slowly. Gustav spawned 41 tornadoes--21 in Mississippi, 11 in Louisiana, 6 in Florida, 2 in Arkansas, and 1 in Alabama. The strongest tornado was an EF2 in Evangeline Parish, Louisiana. Isaac will likely produce 10+ tornadoes. The total damage from Gustav in the U.S. was $4.5 billion (2012 dollars.) I expect Isaac's damage total will be in the $500 million - $4 billion range.

Invest 97L in the Middle Atlantic
A tropical wave (Invest 97L) is located in the Middle Atlantic, about 1250 miles west-northwest of the Cape Verde Islands. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 40% chance of developing by Thursday morning. None of the reliable models foresee that 97L will be a threat to any land areas.

Another tropical wave that moved off the coast of Africa on Sunday is located just southwest of the Cape Verde Islands, and is moving west at 15 mph. Several models develop the disturbance into a tropical depression late this week, and NHC is giving the disturbance a 10% chance of becoming a tropical depression by Thursday morning. The disturbance could begin to affect the Northern Lesser Antilles as early as Saturday night, though our two best models, the GFS and ECMWF, predict the center of the disturbance will pass a few hundred miles north of the islands. The disturbance could be a long-range threat to Bermuda.

Jeff Masters

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Quoting ScottLincoln:

Slidell area... about to head in to work.


Pearl River, but part of the time I live in NOLA in the channel. not today though.
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Quoting julita719:
Sorry if this has already been posted. Do we have any live cams for New Orleans please?
TIA


Check nola.com. They usually have some.
Member Since: June 2, 2008 Posts: 0 Comments: 204
Quoting julita719:
Sorry if this has already been posted. Do we have any live cams for New Orleans please?
TIA

I can do that, I think but how?
Member Since: September 1, 2007 Posts: 0 Comments: 249
Carl Parker of TWC just brought some light to the flooding situation on the SE coast..lots and lots of heavy rain in SC right now..
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Quoting HoustonTxGal:


Is this western shift confirmed? I have seen others mention this in other blogs.


Even though the official direction has been listed as "NW" many times, Isaac has never had a 3 hour sustained motion above 310, or 40 degrees north of west. 45 degrees north of west, or 315 is "true" NW.

Not much difference, but if you add that up over hours and days it moves the storm quite a bit left of a TRUE NW heading.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting zoomiami:


Aren't you still in Mexico?


Not unless they Built a Superedome .

: )

Back in NOLA ...yesterday,late, late.
Member Since: July 3, 2005 Posts: 421 Comments: 127639
Quoting julita719:
Sorry if this has already been posted. Do we have any live cams for New Orleans please?
TIA


here are a few

http://www.nola.com/live/
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Quoting HoustonTxGal:


Is this western shift confirmed? I have seen others mention this in other blogs.


Nope. It's up to the NHC to change track. Guess we'll see if they do soon.
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Quoting southfla:
Here is a generally accepted range of pressures for hurricane categories:

Cat Wind Speed    Pressure        Storm Surge
1    75-95 mph       > 980 mb          4-5 ft
2    96-110 mph     965-979 mb      6-8 ft
3    111-130 mph    945-964 mb     9-12 ft
4    131-155 mph    920-944 mb    13-18 ft
5    > 155 mph         &nbs p; 18 ft


Those storm surge values will not be realized.... the size and speed of the storm are expected to cause larger surges for Isaac.
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This is not good not good at all
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Quoting Elena85Vet:


Don't forget about coastal Mississippi.

They don't get the press.


Isn't that the "Land Mass Between New Orleans and Mobile" (tm)?
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Sorry if this has already been posted. Do we have any live cams for New Orleans please?
TIA
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Quoting Patrap:


There is no Higher ground than here..at 20 Ft.


Audubon Ridge,Uptown NOLA

Weather Station - report

Uptown, New Orleans

Elevation
20 ft


Station Select
Now

Heavy Rain
Temperature
83.0 °F
Feels Like 91 °F


Aren't you still in Mexico?
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Here is a generally accepted range of pressures for hurricane categories:

Cat Wind Speed    Pressure        Storm Surge
1    75-95 mph       > 980 mb          4-5 ft
2    96-110 mph     965-979 mb      6-8 ft
3    111-130 mph    945-964 mb     9-12 ft
4    131-155 mph    920-944 mb    13-18 ft
5    > 155 mph         &nbs p; 18 ft
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Quoting HoustonTxGal:


Is this western shift confirmed? I have seen others mention this in other blogs.


Hopefully west enough to bring some rain and a little cool breeze to H-town.
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Quoting HoustonTxGal:


Is this western shift confirmed? I have seen others mention this in other blogs.


it does not seem important but it is because if this occurs then the populated area of Lafayette will see worse conditions versus a SE LA landfall
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Quoting Patrap:
From my niece on the West Bank of NOLA


Well on the bright side, today is the anniversary of the very first power rangers episode ^-^


You don't say...LOL got me to laugh!
Member Since: September 1, 2007 Posts: 0 Comments: 249
well, about 95 miles in front of the new center fix, directly in the storm's path, a land based observation station recorded a 78mph wind gust an hour ago... at 12:30edt. So around 3pm gusts like this could be coming to the SE part of NOLA. Go to the link to see the wind readings every 6 minutes.

LINK

Member Since: December 28, 2010 Posts: 0 Comments: 134
Quoting LongGlassTube:


That's a good place to be Pat.

My house on the Amite River sits at 10 feet and my parents sits at 6 feet. No levees here.


Good Luck..watcha dem Tall Pines .
Member Since: July 3, 2005 Posts: 421 Comments: 127639
Quoting doubtit:
I went trough Andrew, Katrina and Wilma in Miami, and what you are saying is pure ignorance. So unless you have been through a cat 1, 3 and 5 like I have personally stop trying to scare people. Yes this should be taken seriously as with any storm, but a cat 1 WILL NOT cause as much damage as a cat 3 or 5.


A hurricane is more than just wind. Isaac brings up to 14 feet of surge with him, and a lot of rain. Might turn out to be more devastating than a cat 2, might not.

Dont underestimate this system! Remember Ike was only Cat 2, and a true catastrophe as opposed to a disaster!
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Quoting louisianaboy444:
Models might be showing the Western turn right at the coast now. It will be interesting to see how west it may go. 12Z GFS NAM now both showing this


Is this western shift confirmed? I have seen others mention this in other blogs.
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Quoting seminolesfan:
NOPE, SS Categories are base solely on wind speeds. There are a set of pressure ranges that COULD statistically be relevant to wind speeds, but its not a metric included in Saffir-Simpson ratings.


Yes. While 975 mb is typically associated with a low end cat 2, cases can differ. Isaac is a good example, his size makes it take longer for the winds to catch up. He COULD conceivably reach 100 mph or so if he had time to do so, but he will not.
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From my niece on the West Bank of NOLA


Well on the bright side, today is the anniversary of the very first power rangers episode ^-^

Member Since: July 3, 2005 Posts: 421 Comments: 127639
Adjusted landfall intensities:

970mb 12z Adjusted HWRF in 12 hours.
968mb Linear extrapolation using 4 hour average
971mb 6z Adjusted GFDL
970mb 12z Adjusted GFS
962mb linear extrapolation using 2 hour average...
973mb 00z Adjusted Euro
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting lurkersince2008:
Tornado warning here in Gulfport.


Do you mean tornado watch? No warnings in Gulfport I can find.
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Quoting Patrap:


There is no Higher ground than here..at 20 Ft.


Audubon Ridge,Uptown NOLA

Weather Station - report

Uptown, New Orleans

Elevation
20 ft


Station Select
Now

Heavy Rain
Temperature
83.0 °F
Feels Like 91 °F


That's a good place to be Pat.

My house on the Amite River sits at 10 feet and my parents sits at 6 feet. No levees here.
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big rain in WPB now not good
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Quoting reedzone:


Pressure wise, this IS a Category 2 Storm.
NOPE, SS Categories are based solely on wind speeds. There are a set of pressure ranges that COULD statistically be relevant to wind speeds, but its not a metric included in Saffir-Simpson ratings.
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Beautiful watching the waves break on the railroad track Pensacola Bay. Very heavy winds just on the water--not strong at all inshore.

I hope everyone getting the actual storm will be OK. It goes from beautiful to ugly really fast.
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Quoting kmanislander:


The HH is now sampling the wind field just East of Venice and finding surface winds over 60 MPH. Lots of water will be carried inland with this hurricane.


Yes. That is my biggest worry- Rain & Surge. Hopefully it will move out and not just sit.
Member Since: June 2, 2008 Posts: 0 Comments: 204
Quoting GetReal:
Over the past 30 years SE LA has sustained severe land losses to coastal erosion. Many who are not familiar with the geography of LA, looking at the satellite pics may be under the mistaken impression that Isaac is going to make landfall, when he makes contact with the MS river delta. That would be a mistake. Much of the "land areas" depicted on the satellite pic outlines are actually open waters now.

The remaining area south of NOLA are patchy salt water marshes, and bays. Isaac will have to get as far north as 29.6N and almost 90W before finally reach hard real land.



Amen to that. NOLA is well protected against a storm of this size now. The outlying areas however are more vulnerable than ever.

What is land on a paper map isn't land anymore. Fly over the area and you'd be amazed. More loss will occur from wave action and saltwater invasion from this storm and every storm to come.
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Quoting kmanislander:


I hope you are on high ground.


There is no Higher ground than here..at 20 Ft.


Audubon Ridge,Uptown NOLA

Weather Station - report

Uptown, New Orleans

Elevation
20 ft


Station Select
Now

Heavy Rain
Temperature
83.0 °F
Feels Like 91 °F
Member Since: July 3, 2005 Posts: 421 Comments: 127639
Quoting JasonRE:
Anyone here today in Lafayette? Leaving work at 3 today and they have us open for work tomorrow. I'm NOT coming in. I'd rather lose 8 hours of pay then not be at home watching my home.

What is the general consensus for Lafayette's weather from later tonight until Wednesday night??
Being in Lafayette you are close enough to be concerned, if Isaac comes a little more west then I can see it getting pretty rough, depends where he goes when he makes landfall, but wind damage and flooding is very possible close to your area, I would feel better if I lived in Lake Charles.
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Quoting GOLSUTIGERS:


My hummingbirds were still looking for food this morning. To bad I took them down last night.


I am leaving mine up here as long as possible.
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looks like New Orleans is going too be on the bad side of this
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Quoting Patrap:


Oh I hope so or the Univesre is skewed mucho.

Pat...I need the correct recipe for fresca...this stuff by itself is bland...winds gusting to 30 here..Mandeville
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Quoting freeroam:

I am in Lakeview close to City Park.


Earhart and Broad over here - right by Times Picayune.
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Models might be showing the Western turn right at the coast now. It will be interesting to see how west it may go. 12Z GFS NAM now both showing this
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Quoting dartboardmodel:
Wow! Charleston S.C is flooding big time! It's all over the local news here. People getting rescued, Market Steet in downtown is flooding, interstates are flooding. That stinking hurricane kicked up this huge feeder band waaaay east of the storm. It's pretty bad over here and the rain keeps developing!


Understandably there is a hurricane in the GOM but the Carolinas are having a time here..round two is next for those in eastern NC..we are under a flood watch and I think this round will upgrade it..



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534. VR46L
Quoting reedzone:


Pressure wise, this IS a Category 2 Storm.


But wind speed wise it aint ...and which is more important just sayin...


Anyways the man , himself in rainbow

Member Since: March 1, 2012 Posts: 0 Comments: 6842
It's a bit odd that not one buoy or the weather station at Pilot Town, LA is reporting anything near hurricane force winds. It's a stretch to say this storm is a hurricane.
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Quoting dartboardmodel:
Wow! Charleston S.C is flooding big time! It's all over the local news here. People getting rescued, Market Steet in downtown is flooding, interstates are flooding. That stinking hurricane kicked up this huge feeder band waaaay east of the storm. It's pretty bad over here and the rain keeps developing!
Flash flooding situation in Charleston.

"Reports from local
media... law enforcement and the public indicate numerous roads
flooded and impassible in downtown Charleston as well as parts of
west Ashley and Mount Pleasant. In addition... there have been dozens
of people rescued from their vehicles and a report of water entering
the lobby of the stern center at the College of Charleston."
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Well guys I will check in later please everyone be safe..
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North breeze starting to pick up here north of Lake Charles, LA. Breezy with gusts.
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Member Since: July 3, 2005 Posts: 421 Comments: 127639
My Met class is cancelled today, so it looks like I'll do a little studying of my own with Issac. If I was on the gulf I would be very happy that he's taking his time to build a proper eyewall, we are getting one of the best scenarios with Isaac saying on the line of TS/HU. The gulf can produce monsters as we all know.

Stay safe out there if you're near Isaac!
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Quoting Caner:
If this was a cat 3 or 4 on this exact same path, NOLA would be utterly devastated.

This is a worst possible case scenario path wise for a land falling hurricane in NOLA...

Just look at that eye lining up to pile water into Lake Pontchartrain...

NOLA should thank their lucky stars they are getting off with a tropical storm today hehe.



Don't forget about coastal Mississippi.

They don't get the press.
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975 mb, 75 mph cat 1, amazing. And on the opposite spectrum, remember Danny from 2003? 1000 mb, 75 mph cat 1.

Hurricane Danny

Danny had special circumstances for his pressure.
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Man folks on channel 8 at waterfront in New Orleans was showing people playing around the waters edge police just showed up to get them away good job
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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